1. Field of the Invention
The present invention relates to a method and to a system for transmitting (notably by radio waves) data to a remote station, allowing reacquiring of data acquired by a data collection equipment comprising large number of local collection stations. The method preferably uses compression techniques in order to optimize the use of available transmission channels.
2. Description of the Prior Art
Various seismic data transmission systems are used to connect local acquisition units to a central station, either directly or via intermediate stations provided complex local units concentration or control functions. The links can bc provided by cables, radio links, possibly via one or more relays, or the cable links and the radio links can be combined as mentioned for example in French Patent 2,720,518, European Patents 0,594,477 and 0,275,781, and U.S. Pat. Nos. 4,815,044, 4,979,152, 4,583,206 or 4,908,206 filed by the assignee.
The assignee's European Patent 0,275,781 notably mentions the use of seismic acquisition units fitted with two transmission channels, one with a relatively high transmission rate, the other with a bandwidth that can be relatively narrow according to the local availability of the transmission frequencies, more readily available within the framework of the radioelectric emission regulations in force. The seismic data collected during the successive cycles arc stored on a mass storage in each of the units and intermittently transferred to a central control and recording station. In order to allow the operator in the central station to check that data acquisition by each acquisition unit is progressing normally, a partial data transmission is performed, which turns out well with a transmission channel with a relatively narrow bandwidth.
French Patent 2,692,384 and corresponding U.S. Pat. No. 5,550,787, also filed by the assignee, mentions using seismic acquisition units notably provided with a processed having specialized signal processing which performs many geophone and acquisition chain element controls and seismic trace preprocessings which were previously performed at the central station after transmission, thus allowing a considerable decrease in the volume of data to be required.
The current trend, notably within the scope of 3D seismic exploration methods, spreads out, in a zone to be explored, onshore, offshore or in coastal areas, often over several square kilometers, hundreds or even thousands of seismic receivers. The volume of the data to be collected and transmitted increases constantly. In order to prevent transmission problems from braking the evolution of seismic systems, the trend consists in using data compression processes compatible with the geophysicists' specific requirements.
If for example the problem of real-time repatriation to a central station of 1000 seismic "traces", each sampled every 2 ms during a recording cycle n is considered, it is necessary to transmit, during the progress of the next cycle n+1, a cross global transmission rate of 14.4 Mbit/s (if the error correction and detection bits are taken into account) to be distributed among 36 radio channels. If a TFM type modulation system allowing a 400 Kbit/s transmission rate in a 500 kHz bandwidth is used therefore, a global bandwidth of 18 MHz is necessary. This condition is very difficult or even impossible to satisfy in practice for many reasons. Sufficient transmission licenses first have to be obtained from the local bodies controlling frequency allocation to users. The radio environment and the weather conditions are likely to disturb transmissions and thunderstorms, static discharges, etc, also have to be taken into account. The configuration of the sites crossed and local link difficulties can also constitute an additional problem. For all these reasons, the bandwidth that is actually available is often much narrower than that required for real-time data transmission.
In addition to the specifically radio constraints, the fact that the seismic acquisition velocity is also imposed by the exploration conditions also has to be taken into account. In intermediate zones running along shores, the seismic sources are for example air guns towed underwater by one or more boats progressing at constant speed which impose a particular "shooting" rate and periodical seismic session interruptions during the time requited for U-turns and for realignment along a new profile.
In order to prevent transmission problems from braking the evolution of seismic systems, the trend consists in using data compression processes compatible with the geoplysicists' specific requirements.
French Patent 2,757,641 filed by the assignee relates to a two-stage compressed seismic data transmission method intended for transmission, to a central station, of the seismic signals received by seismic receivers and acquired by collection units placed in the field, which is well-suited for turning the irregular distribution of the seismic data collection peaks to best account, in the usual practice of seismic exploration operations. This method allows performing control transmissions intended to check the smooth running of acquisition operations by means of each of the acquisition units in the field as well as complete transmissions of seismic data to a central control station, so as to recover without loss all the seismic traces transmitted while limiting as much as possible the transmission times required therefore.
A invention comprises two-stage transmission of the seismic data having, during a first stage, compressed data transmission allowing control of the quality of operation of the local units and, during a second stage, (compressed or not) data transmission and recording without seismic data losses at the remote station.
The previous method comprises for example local recording of the data, data compression with a selected compression rate, followed by a first state of transmission of these compressed data, determination of the difference between the data recorded locally and the corresponding compressed data, a later stage of transferring this difference and reconstitution, at the central station, of the seismic data collected by each local collection unit by combining the data received during the two transmission and transfer stages.
During the first stage, a compression whose rate is suited to the duration of a transmission window placed during time intervals between successive seismic emission-reception cycles and to the transmission rate of the channel used is applied to each seismic trace. At least part of each seismic trace is thus transmitted, which allows an operator at the central station to check that recording goes off correctly in each collection unit. During the second stage, at the end of the seismic session in progress for example, the seismic traces are reconstituted in the central station with all the precision required.